1. Field of the Invention
This invention pertains to processes for preparing dihaloethenylcyclopropanecarboxylate insecticides, more specifically to a process for preparing a 3-(2,2,2-trichloroethyl)-2,2-dimethylcyclopropane-1-carboxylate predominantly in its cis isomer form as an intermediate in the production of dichloroethenylcyclopropanecarboxylate insecticides predominantly in their cis isomer form.
2. Description of the Prior Art
Belgian Pat. No. 833,278 describes a process for making dihaloethenylcyclopropanecarboxylates. The patent discloses, inter alia, that the base induced dehydrohalogenation of certain 4,6,6,6-tetrahalohexanoates can be controlled by varying the reaction conditions to give any one of four different products including a trihaloethylcyclopropanecarboxylate and a dihaloethenylcyclopropanecarboxylate exemplified respectively by compounds Z and C below wherein R is a defined alcohol residue. ##STR1## Under certain conditions compound Z is an intermediate in the reaction leading to compound C. These two compounds correspond to compounds Z and C described in the instant application, and differ from them mainly in the relative amounts of cis and trans isomers in each. The process of the Belgian patent reportedly produces compound C and, presumably, the compound Z intermediate, wherein the trans isomer comprises from 50% to 90% of the isomer mixture. The process of the present application produces compounds Z and C wherein the cis isomer comprises more than 50% of the isomer mixture.
To obtain compound C by the process of the Belgian patent, a 4,6,6,6-tetrahalohexanoate is allowed to react with at least 1.5 molar equivalent of base (sodium and potassium hydroxide, alkali metal alkoxides, sodium hydride, and sodium napthalide are disclosed) in a solvent (alcohols and ethers are disclosed) at a temperature of from -30.degree. C. to 200.degree. C. When the reaction is conducted at 25.degree. C. to 50.degree. C. in the presence of sodium or potassium tert-butoxide in benzene, dioxane, dimethylformamide, tetrahydrofuran, tert-butyl alcohol, or a mixture of tert-butyl alcohol with benzene, the product is reported to be compound Z. Compound Z can be converted to compound C by further treatment with base.
It was reported in the Belgian patent that the ratio of cis to trans isomers in the product compound C was, to a certain extent, a function of the reaction temperature, with the trans isomer being favored at higher temperatures. In no case, however, was the cis isomer reported to be produced in excess of the trans, even at low tempertures. The highest cis/trans ratio disclosed for compound C is 1:1. In that case, ethyl 4,6,6,6-tetrachlorohexanoate was treated at 0.degree. C. for two hours with 2.2 equivalents of sodium tert-butoxide in tetrahydrofuran to give the 50/50 mixture of cis and trans isomers of ethyl 3-(2,2-dichloroethenyl)-2,2-dimethylcyclopropane-1-carboxylate.
Similarly, Japanese Patent Application No. 122242/74 (OPI No. 59839/76) discloses a process for preparing either compound Z or compound C above. There R is defined as hydrogen or lower alkyl. According to the Japanese application when a 4,6,6,6-tetrachlorohexanoate is treated with a base at from 0.degree. C. to 25.degree. or 30.degree. C. for up to one or two hours the product is compound Z. To prepare compound C, the reaction is conducted at a higher temperature and for a longer period of time. The process is disclosed as producing exclusively the trans isomer. Suitable bases include various alkyl lithium compounds and alkali metal alkoxides, amides, and hydroxides. The type of solvent used is reported not to be critical; alcohols, ethers, aromatic hydrocarbons, and liquid ammonia are disclosed. Aromatic hydrocarbons are said to be generally used.
German Offenlegungschrift No. 2605398 discloses generically, inter alia, the process exemplified by the following chemical equations. ##STR2## According to the disclosure, the dehydrochlorination of the tetrachloro ketone 1 is effected by treatment with base (alkali metal hydroxides and alkoxides are disclosed) at from -20.degree. to 120.degree. C. in a suitable solvent (water, water plus an alcohol, alcohols, ethers, and aromatic hydrocarbons are disclosed). The nature of the product and its isomer makeup are reported to depend upon the reaction temperature and other reaction conditions such as the nature of the base used. Generally, when the reaction is conducted in a lower alcohol at from -20.degree. to 25.degree. C. the product is compound 2 wherein the cis isomer predominates. At higher temperatures the trans isomer predominates and the product is compound 2 and/or compound 3. Depending upon the reaction conditions it is reported that one can obtain product having a cis/trans ratio of from 1:9 to 9:1. Although this process pertains to dehydrochlorinations involving ketones rather than carboxylic acid esters as in the Belgian and Japanese references above, the teaching that the cis/trans isomer ratio in the product is temperature dependent is consistent with the Belgian disclosure. The process is further described and expanded in Synthetic Pyrethroids, ACS Symposium Series, No. 42, M. Elliott, Ed., American Chemical Society, Washington, D.C., 1977, chapter 4. Compound 2 is described as being a key intermediate for the preparation of the acid corresponding to compound C having a controlled cis/trans isomer ratio.
The foregoing prior art teaches or suggests that temperature and base are the critical factors in determining the cis/trans distribution when a 4,6,6,6-tetrachlorohexanoate is converted by dehydrohalogenation to either compound Z or compound C.
It has now been found that in systems employing an alkali metal tert-alkoxide as the base, for any given temperature up to about 30.degree. C., the ratio of cis to trans isomers in the product, compound Z or compound C, is unexpectedly and surprisingly increased by employing a solvent system comprising a non-polar or slightly polar solvent and a selected polar aprotic cosolvent. Moreover, it has been determined that the cis/trans ratio in the intermediate, compound Z, will generally control the cis/trans ratio in compound C when the latter is prepared via compound Z in accordance with prior art methods.